A Study Of Magnetic Field Controlled Complex Fluid Flow And Heat Transfer Based On Lattice Boltzmann Method

Complex fluids composed of multiphase and multicomponent have complex intrinsic structure relationships and rich and diverse dynamical characteristics,which have important applications in flow and heat transfer control in the fields of mechanical processing,digital microfluidic systems,biomedical and energy power,etc.The realization of accurate control of them has received extensive attention from many researchers.Magnetic field control technology has significant advantages in precise control of flow and heat transfer in complex fluid due to its flexibility,high efficiency and low energy consumption.However,the complex nature and dynamics of complex fluids make it possible to generate instability in magnetic control,which greatly affects the control accuracy.Therefore,in order to achieve more effective and precise control,a deeper understanding of the physical mechanism behind the flow and heat transfer of complex fluids controlled by magnetic fields is required.Based on this,it is of great importance to develop mesoscopic simulation models to reveal these mechanisms from the perspective of cross-scale mesoscopic studies.Since the current Lattice Boltzmann(LB)model has limitations in studying the flow and heat transfer characteristics of nanofluids and magnetic fluids in complex fluids,and there is a lack of more simple and effective mesoscopic models in the simulation of magnetic multiphase flow,the extension and coupling development of the LB model is carried out in this work,and firstly,the accuracy of the model is verified by benchmark cases and the results of previous work,and the models are found to have good computational accuracy.Then,the proposed model is applied to study the flow and heat transfer characteristics of nanofluid under the effect of magnetic field,as well as the interfacial evolution and dynamic characteristics of magnetic fluid.In order to achieve good computational accuracy,efficiency and stability,this work extends the nonorthogonal multi-relaxation time(MRT)LB model for multiphysics field simulations and verifies that the computational accuracy of the nonorthogonal MRT-LB is better than the orthogonal MRT-LB model.Then,this model is utilized to study the natural convection heat transfer characteristics of nanofluid in a porous corrugated triangular cavity under the effect of magnetic field.The effects of Hartmann number on the heat transfer performance are analyzed when the heat transfer mode in the cavity is dominated by conduction and convection,respectively,and the mechanism of the effects is clarified.The variation of the weakening effect of the magnetic field on the heat transfer performance with the Rayleigh and Darcy numbers is also investigated,and it is found that the weakening effect of magnetic field on heat transfer performance increases not always with the increase of convection intensity.In addition,by analyzing the effects of the structural parameters of the corrugated walls on the heat transfer performance in the cavity,it is found that there exist optimal structural parameters to achieve the optimal heat transfer performance.Finally,the effects of the volume fraction of nanoparticles on the heat transfer performance and the maximum stream function in the cavity are analyzed,the mechanism of the effects is revealed.To achieve the accurate resolution and dynamic evolution of the phase interface and the wetting characterization of the solid surface in magnetic multiphase flow more simply and efficiently,and to make the model more extensible,the coupling of the multicomponent multiphase pseudopotential LB model and the magnetic field LB model is firstly proposed and realized in this work,which is named as the multiphase LB coupling model.Then,the model is applied to study the morphological evolution and dynamics of two asymmetrically arranged bubbles in a magnetic fluid under the effect of a horizontal uniform magnetic field.The effects of the magnetic Bond number and the spacing between the bubbles on the morphological evolution and dynamics of the bubbles are analyzed,and the influence patterns are revealed.It is found that the bubbles deform irregularly,have four representative dynamical modes,and yield oscillation in the direction parallel and perpendicular to the magnetic field.Then,the mechanism behind the above phenomena is analyzed,and the relationship between the magnetic field intensity gradient and the pressure gradient is revealed.It is found that the magnitude and distribution of the pressure gradient around the bubble interface is the dominant factor in producing the above phenomena.In order to extend the mesoscopic model for simulating the wetting dynamics of magnetic fluid droplets,the multiphase LB coupling model is extended to simulate and study the wetting dynamics of magnetic fluid droplets on hydrophobic surfaces under the effect of non-uniform magnetic field and gravity.The effects of magnetic Bond number and gravity Bond number on the wetting dynamics of magnetic fluid droplets are analyzed,and the influence patterns are revealed.The morphological structure of the droplets generating bulges and concavities as well as the process of generating oscillations are observed.Subsequently,the mechanism behind the morphological evolution and dynamics of magnetic fluid droplets is analyzed with emphasis.It is found that the curvature of the phase interface significantly affects the magnetic field intensity distribution and pressure distribution around the interface,which in turn affects the surface pressure on the phase interface.The unbalanced surface pressure dominates the morphological evolution of magnetic fluid droplets.Finally,the splitting dynamics of magnetic fluid droplets is analyzed in depth to reveal the mechanism influencing the splitting.